Infrared sensors have been used to attempt to measure small temperature gradients in industrial materials and biological tissue. Measurements of temperature distributions in human tissue to detect tumors, for example, must map the surface temperature accurately so that any contribution from internal tumors can be properly detected. Infrared sensors can become unreliable because external conditions such as air flow between the sensors and the sensed region can corrupt the temperature measurements.
Conventional temperature transducers include thermocouples, resistance-temperature detectors, thermistors, and integrated sensors. Each of the above types of transducers, except for the thermocouples, is an active device requiring an external power supply. The thermocouple is a passive device which senses temperature using the thermoelectric principle that when two dissimilar electrically conductive materials are joined, an electrical potential (voltage) is developed between these two materials. Each such individual material can be called a thermoelectric material. The voltage between the materials varies with the temperature of the junction (joint) between the materials. Thermocouples which are useful in practical applications provide approximately linear changes in voltage corresponding to changes in temperature over a useful temperature range. Useful temperature ranges are generally several hundred degrees Celsius and typically occur from absolute zero to the melting point of one or more of the two materials (or, if applicable, a lower temperature that causes the pair of materials to function less linearly).
The voltage varies according to the conductor materials used for the two metals. The voltage cannot be measured directly because the leads to a voltmeter establish a new thermoelectric circuit when they are coupled to the thermocouple leads. Accurate voltage measurements across the thermal junction can be made by instrumentation systems by bringing the thermocouple leads through an isothermal junction block to equilibrate the temperatures of the leads.
Conventional thermocouples are generally fabricated by welding two dissimilar wires. When precise measurements which do not affect the thermal environment are required, the wires are reduced in diameter to eliminate effects of thermal conductivity along the wires, and the thermocouple becomes more fragile. Using conventional thermocouples to measure temperature gradients across a large area involves the fabrication of a thermocouple array connected to a voltmeter with a sequential scanner such as a reed relay scanner. Mechanical support is required for the fragile thermocouple wires. These thermocouple arrays and mechanical supports are not capable of conforming to irregularly-shaped surfaces. Other limiting factors are the precision, accuracy and uniformity of the thermocouple sensors and their thermal junctions created by physical joining or welding.